The subject matter of this disclosure relates generally to rotor blades for a wind turbine, and more particularly to a leading edge extension (LEX) in combination with a trailing edge strake (TES) as a means of reducing aerodynamic losses and enhancing torque extraction from the root section of a wind turbine blade.
Rotor blades are primary elements of wind turbines for the conversion of wind energy into electrical energy. The working principle of the rotor blades resembles that of airplane wings. A cross-section of a typical blade, during operation thereof, enables air to flow along both sides of the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade.
In addition, an attached-flow region has a mainly laminar flow along an outer surface area of the blade. In contrast, a detached-flow region in the wake of flow separation has a more turbulent flow. Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, in-flow atmospheric turbulence) and characteristics of the blade (e.g. airfoil sections, blade chord and thickness, twist distribution, pitch angle, etc).
The lift force is predominantly created in the attached-flow region, whereas the detached-flow region leads to an increase in drag force, mainly due to a pressure difference between the upstream attached-flow region and the downstream detached-flow region.
The force component used to produce electrical power is a portion of the lift force acting as torque on the rotor main shaft. Hence, in order to increase the energy conversion efficiency during normal operation of the wind turbine, it is desired to maximize the lift force. On the other hand, it is generally desired to minimize the drag force. To this purpose, it is advantageous to increase the attached-flow region and to reduce the detached-flow region by having the flow separation near a trailing edge of the blade, i.e. in a downstream region of the blade. Also, it is generally desired to have a stable flow separation, e.g. in order to increase the working stability or to decrease noise generation.
Current manufacturing practices generally prevent the attainment of desired or ideal angles of attack in the root region of wind turbine blades due to constraints on blade twist. The root region airfoil sections, as a result, are at very high angles of attack resulting in separation, poor lift to drag (L/D) ratio, and low lift. Interpolated sections from the first designed airfoil to the cylindrical section of the root are also not aerodynamically optimized, resulting is less than optimal aerodynamic performance.
In view of the foregoing, there is a need for an airfoil structure that overcomes the foregoing disadvantages to provide more optimal aerodynamic shapes in the airfoil root regions.